It is well known to use materials which abrade readily to form seals between a rotating part and a fixed part, whereby the moving part erodes a portion of the abradable material to form a seal having a very close tolerance. An important application of abradable seals is in gas turbines, in which a rotor consisting of a plurality of blades mounted on a shaft rotates inside a shroud. By minimizing the clearance between the blade tips and the inner wall of the shroud, it is possible to reduce leakage of gas across the blade tip and thereby maximize turbine efficiency. This may be achieved by coating the inner surface of the turbine shroud with an abradable material, so that rotation of the blades and contact with inner surface causes wear of the abradable material to form grooves in the abradable coating. As the turbine blades rotate, they expand due to centrifugal effects as well as heat expansion. The differential expansion rate between the rotor and the inner shroud results in the tips of the blades contacting the abradable material and carve precisely defined grooves in the coating without contacting the shroud itself. In this way, an essentially custom-fitted seal is provided for the turbine.
Typically, high temperature abradable coatings comprise a continuous porous ceramic coating, e.g., yttria stabilized zirconia, applied to the shroud. The blade tip is coated/reinforced with abrasive grits such as cubic boron nitride (cBN). Drawbacks of this system are the short life of the cBN at these high temperatures and the complexity of the tipping process. See, for example, U.S. Pat. No. 6,194,086 or 5,997,248.
U.S. Pat. No. 6,251,526B1 describes profiled abradable ceramic coating systems, in which a porous ceramic coating is deposited onto a substrate with a profiled surface, e.g., a metal grid brazed onto the substrate surface (FIG. 1), to form an abradable profiled surface. The profiled surface can be made in different forms as described in U.S. Pat. No. 6,457,939B21. However, a drawback of this method is that since the grid is brazed onto the substrate permanent damage can result to the shroud upon profiling.
A need exists for an abradable coating system that will not require blade tipping and will not have to be profiled through a destructive method such as brazing a grid structure. The present invention seeks to fill that need.